This invention relates generally to electron emitters. In particular, the invention relates generally to field-enhanced Metal-Insulator-Semiconductor (MIS) or Metal-Insulator-Metal (MIM) electron emitters, hereinafter collectively referred to as FEMIS.
Electron emission technology exists in many forms today. For example, cathode ray tubes (CRT) are prevalent in many devices such as TVs and computer monitors. Electron emission plays a critical role in devices such as x-ray machines and electron microscopes. In addition, microscopic cold cathodes can be employed in electron-beam lithography used, for example, in making integrated circuits, in information storage devices such as those described in Gibson et al, U.S. Pat. No. 5,557,596, in microwave sources, in electron amplifiers, and in flat panel displays. Actual requirements for electron emission vary according to application. In general, electron beams need to deliver sufficient current, be as efficient as possible, operate at application-specific voltages, be focusable, be reliable at the required power densities, and be stable both spatially and temporally at a reasonable vacuum for any given application. Portable devices, for example, demand low power consumption.
Metal-Insulator-Semiconductor (MIS) and Metal-Insulator-Metal (MIM) electron emitter structures are described in Iwasaki, et al, U.S. Pat. No. 6,066,922. In such structures with the application of a potential between the electron supply layer and the thin metal top electrode, electrons are 1) injected into the insulator layer from the electron supply layer (metal or semiconductor), 2) accelerated in the insulator layer, 3) injected into the thin metal top electrode, and 4) emitted from the surface of the thin metal top electrode. Depending upon the magnitude of the potential between the electron supply and thin metal top electrode layers, such emitted electrons can possess kinetic energy substantially higher than thermal energy at the surface of the thin metal film. Hence, these emitters may also be called ballistic electron emitters.
Shortcomings of MIS or MIM devices include relatively low emission current densities (typically about 1 to 10 mA/cm2) and poor efficiencies (defined as the ratio of emitted current to shunt current between the electron supply layer and the thin metal electrode) (typically approximately 0.1%).
Electrons may also be emitted from conducting or semiconducting solids into a vacuum through an application of an electric field at the surface of the solid. This type of electron emitter is commonly referred to as a field emitter. Emitted electrons from field emitters possess no kinetic energy at the surface of the solid. The process for making tip-shaped electron field emitters, hereinafter referred to as Spindt emitters, is described in C. A. Spindt, et al, xe2x80x9cPhysical Properties of Thin-Film Field Emission Cathodes with Molybdenum Conesxe2x80x9d, Journal of Applied Physics, vol. 47, No. 12, December 1976, pp. 5248-5263. For a Spindt emitter, the electron-emitting surface is shaped into a tip in order to induce a stronger electric field at the tip surface for a given potential between the tip surface and an anode; the sharper the tip, the lower the potential necessary to extract electrons from the emitter.
The shortcomings of Spindt emitters include requiring a relatively hard vacuum (pressure  less than 10xe2x88x926 Torr, preferably  less than 10xe2x88x928 Torr) to provide both spatial and temporal stability as well as reliability. Furthermore, the angle of electron emission is relatively wide with Spindt emitters making emitted electron beams relatively more difficult to focus to spot sizes required for electron-beam lithography or information storage applications. Operational bias voltages for simple Spindt tips are relatively high, ranging up to 1000 volts for a tip-to-anode spacing of 1 millimeter.
In other words, combining high current density, stability, and reliability in one device has been difficult, if not impossible, with previous designs of electron emitters.
In one aspect, an embodiment of a FEMIS electron emitter may include an electron supply structure. The electron supply structure is such that at least one protrusion, preferably a plurality of protrusions, may be formed on a top side of the electron supply structure. The electron supply structure may be formed from a conductive substrate and optionally an electron supply layer formed above the conductive substrate. The electron emitter may also include an insulator formed above the electron supply structure and protrusion(s). In this manner, any given protrusion becomes internal to the electron emitter structure. The electron emitter may further include a top conductive layer formed above the insulator.
The FEMIS electron emitter may be such that a portion of the insulator above any given protrusion is thinner than a portion of the insulator that is above the relatively flat region of the electron supply structure. Additionally the insulator may be shaped to curve inward on both lower and upper sides near any given protrusion, i.e., the insulator may be hourglass-shaped locally. The insulator can also be substantially conformal to the electron supply layer, i.e., have the shape of the surface of the electron supply layer including protrusions prior to the formation of the insulator.
In another aspect, an embodiment of a method of forming an FEMIS electron emitter may include forming an electron supply structure. The electron supply structure is such that at least one protrusion, preferably a plurality of protrusions, may be formed on a top side of the electron supply structure. The electron supply structure may be formed from a conductive substrate and optionally from an electron supply layer formed above the conductive layer substrate. The method may also include forming an insulator above the electron supply structure and protrusion(s). In this manner, any given protrusion becomes internal to the electron emitter structure. The method may further include forming a top conductive layer above the insulator.
Certain embodiments of the present invention may be capable of achieving certain aspects. For example, because any given protrusion is internal to the emitter structure, it is not exposed to vacuum. Thus, the emitter is relatively insensitive to the contaminants that may exist in the vacuum, a situation that helps to prolong the life and efficiency of the emitter as well as to promote the spatial and temporal stability of the electron beam emitted. Consequently, vacuum requirements can be substantially relaxed.
Moreover, the shape of any given protrusion coupled with a relatively thin insulator gives rise to an enhanced electric field at that protrusion when a voltage is applied between the conductive substrate and the top conductive layer. Relatively low operational voltages are therefore possible with FEMIS electron emitters. The electric field away from any given protrusion is relatively low thereby improving the reliability of the entire emitter structure. Current densities and efficiencies from FEMIS electron emitters can be relatively high. The electron beam from a FEMIS emitter is also relatively easy to focus.